The invention relates to a filter having an automatically controlled variable cut-off frequency in which the cut-off frequency is automatically varied in accordance with an input signal frequency and in which the output is maintained at a predetermined constant level, and is more particularly concerned with a device for measuring the period or frequency of an input signal by use of such a filter.
Where it is desired to determine the period of an input signal, a gate signal may be derived from the input signal so as to correspond with one period thereof, and may be used to gate reference pulses supplied to a counter. If noises are superimposed on the input signal, there will result a change in the width of the one-period gate signal which is derived therefrom, causing an error in the determination of the period. The influence of such noises can be avoided by passing the input signal through a filter which eliminates frequencies higher than the frequency of the input signal before it is supplied to a period determining apparatus. However, the frequency of the input signal is unknown generally, which explains the need to determine its period, thus precluding a proper choice of the cut-off frequency of the filter which is used to eliminate the noises. In order to permit the use of a filter having a cut-off frequency which varies with the frequency of an input signal, it has been proposed to produce a pulse having a definite width defined by zero crossover points of the input signal, which pulse is rectified to convert the input signal frequency into a voltage level so that the voltage obtained may be compared against a plurality of reference voltages in order to permit a selection of one of filters having mutually different cut-off frequencies. The proposed arrangement requires a relatively complex frequency-to-voltage converter and also requires a number of filters which must be provided. A reduction in the number of filters may result in the failure to eliminate the noises.
It is an object of the invention to provide a filter having an automatically controlled variable cut-off frequency in which the cut-off frequency is automatically varied with the frequency of an input signal by using a simple arrangement.
It is another object of the invention to provide a filter having an automatically controlled variable cut-off frequency in which the cut-off frequency is automatically varied with an input signal frequency and the output level is maintained constant to facilitate the design of following circuitry.
It is a further object of the invention to provide a filter having an automatically controlled variable cut-off frequency in which the cut-off frequency is automatically varied with an input signal frequency and the output level is maintained constant, but can be changed.
It is an additional object of the invention to provide a filter having an automatically controlled variable cut-off frequency which can be used as a variable level regulator.